Such a method is known from European patent application EP 0424120, Japanese patent application JP 2001124529A and from International patent application WO 00/49273.
In the method known from Japanese patent application JP 2001125529A a distributed strain and displacement sensing fiber optical cable is bent into a U-shape and lowered into a underground borehole by means of a weight suspended from the U-folded nose section of the optical fiber.
In the method known from International patent application WO 00/49273 a U-folded distributed temperature sensing fiber optical cable is inserted into a coiled tubing within an underground borehole by connecting the U-folded mid-section of the fiber optical cable to a plug which is bull headed by pumping fluid though the tubing to carry the plug and the fiber optical cable to the bottom of the well.
The known U-shaped, double ended, distributed sensing fiber optical assemblies are inserted into guide conduits that are formed by a well casing and coiled well tubing having an internal width which is typically several centimeters and they are not configured to be installed in small diameter guide conduits.